A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. In a two-stroke cycle, opposed-piston (“OP2S”) engine, two pistons are disposed crown-to-crown in the bore of a cylinder for reciprocating movement in opposing directions along the central axis of the cylinder. The cylinder has longitudinally-spaced inlet and exhaust ports formed in the cylinder sidewall near respective ends of the cylinder. Each of the opposed pistons controls a respective one of the ports, opening the port as it moves toward a bottom dead center (BDC) location during a power stroke (also called an expansion stroke), and closing the port as it moves from BDC toward a top dead center (TDC) location during a compression stroke. One of the ports provides passage for the products of combustion out of the bore, the other port serves to admit pressurized air into the bore; these are respectively termed the “exhaust” and “intake” ports (in some descriptions, intake ports are referred to as “air” ports or “scavenge” ports).
In most instances, OP2S engines operate according to the compression-ignition principle. Early in a compression stroke, pressurized air (“charge air”) enters the bore of a cylinder through the intake port, where it is swirled, agitated, and compressed between the end surfaces of the two pistons as they move from BDC toward TDC. Fuel directly injected into the cylinder between the approaching piston end surfaces mixes with the turbulent charge air. The fuel is ignited by the heat of the compressed air, and combustion follows. The fuel is provided by an engine fuel handling system that includes one or more fuel injectors mounted to the cylinder. Typically, the injectors are located between TDC locations of the piston end surfaces. In some cases, an OP2S engine may include electrical means (a spark plug, a glow plug, a laser) for ignition of the air/fuel mixture.
In an OP2S engine, near the end of a power stroke, charge air entering a cylinder through the intake port displaces exhaust gas flowing out of the cylinder through the exhaust port. Thus gas flows through the cylinder in one direction (“uniflow”)—from intake port to exhaust port. A continuous positive pressure differential must exist from the intake ports to the exhaust ports of the engine in order to maintain the desired unidirectional flow of gas through the cylinders. Without this continuous positive pressure differential, combustion deteriorates and may fail. At the same time, a high air mass density must be provided to the intake ports because of the short time that they are open. This requires pumping work in an OP2S engine, which is unassisted by a dedicated pumping stroke as in a four-stroke cycle engine.
In an OP2S engine, the pumping work to maintain a unidirectional flow of gas is done by an air handling system (also called a “gas exchange” system) which moves fresh air into, and transports combustion gases (exhaust) out of, the engine's cylinders. The pumping work may be done by one or more gas-turbine driven compressors (e.g., a turbocharger) and/or a mechanically-driven pump, such as a supercharger (also called a “blower”). For example, a compressor may be located upstream or downstream of a supercharger in a two-stage pumping configuration. The pumping arrangement (single stage, two-stage, or otherwise) drives the scavenging process, which is critical to ensuring effective combustion, increasing the engine's indicated thermal efficiency, and extending the lives of engine components such as pistons, rings, and cylinder.
In many instances, the air handling system of a uniflow-scavenged, two-stroke cycle, opposed-piston engine is equipped to reduce NOx emissions produced during combustion by recirculating exhaust gas through the ported cylinders of the engine through an exhaust gas recirculation (EGR) loop.
In a typical arrangement, the air handling system includes a charge air channel that transports pressurized air to the intake ports of the engine, an exhaust channel that transports exhaust gasses from the engine's exhaust ports, and at least one turbocharger with a compressor in the charge air channel and a turbine in the exhaust channel. In an example, called a “twin-charging” configuration, the charge air channel may further include a supercharger downstream of the compressor, between the compressor outlet and the intake ports of the engine. In these instances, the air handling system may be equipped with an EGR loop including a channel having an inlet in the exhaust channel and an outlet in the charge air channel. In a high pressure EGR construction, the EGR channel inlet is connected to the exhaust channel upstream of the turbine, between the exhaust ports of the engine and the inlet of the turbine, and the EGR channel outlet is connected to the charge air channel downstream of the compressor, between the outlet of the compressor and the inlet of the supercharger. A valve in the EGR channel enables control of the level of gas flow through the EGR loop.
Use of EGR to control emissions is based upon the premise that gas transported through the EGR channel flows from the exhaust channel to the charge air channel. However, this direction can be maintained only so long as engine gas pressure is higher at the EGR channel inlet than at the EGR channel outlet. If engine gas flows in the reverse direction through the EGR channel (from the outlet to the inlet), there may be detrimental effects on engine performance and emissions. In OP2S engines that use a twin-charging configuration including a supercharger and a turbocharger, and that are equipped with a high pressure EGR channel, engine gas pressure across the channel is higher at the inlet than at the outlet under typical operating conditions, and thus engine gas (exhaust) will normally flow in the correct direction through the EGR channel, from the exhaust channel to the charge air channel. There are, however, instances where gas flow through the EGR channel is susceptible to reversal in an OP2S engine.
During steady state performance, operational parameters of an OP2S engine change slowly. Thus when the engine propels a vehicle on a highway at a steady speed, the recirculation of exhaust gas via an EGR loop can be maintained at a slowly-changing pace. This translates to stable control with enough time to optimize engine performance in terms of emissions. However, vehicle operation often subjects the engine to quick changes in operating conditions. Such changes may include sudden demands for torque or fuel, especially in urban driving or during operation in industrial conditions. Such demands may come from abrupt changes of accelerator pedal position, acceleration and deceleration, switching accessories (like air conditioning) on or off, pulling a trailer, climbing a hill, and so on. A sudden change in demand for torque or fuel associated with an abrupt change in engine load or engine speed is referred to as a “transient event.”
It is desirable to limit the production of emissions during all phases of engine operation. However, during a quick change in engine operating conditions, a limiting factor for OP2S engine response may be defined by how rapidly the air handling system can change the flow of gas through the engine while keeping engine emissions under control. A problem in this regard concerns a change in direction of gas flow through the EGR channel as may happen in response to a transient event causing a sudden reduction in fuel demand while the engine is heavily loaded. One example is a “tip-out” when a vehicle driver quickly removes their foot from the accelerator pedal after pushing it down all the way to accelerate onto a freeway. A “ramp-down” is another example. In these situations the sudden reduction in fuel reduces combustion, which suddenly lowers exhaust pressure. However, charge air flow inertia principally attributable to turbo lag and transport delay in the air handling system can result in a period of time, on the order of seconds in some instances, where the gas flow through the EGR channel reverses because compressor outlet pressure is higher than exhaust pressure.
Therefore, it is desirable that the air handling system of an OP2S engine prevent reversal of gas flow through the EGR channel as may occur during a change in engine operating condition such as, for example, a transient event.